1. Field of the Invention
This invention relates to a process for producing fine spherical silica, and, particularly, to a process for producing fine spherical silica containing only a very small amount of impurities and having a particle size of 100 nm or less.
2. Description of Prior Art
Known processes for producing fine spherical silica include the following:
(1) A process in which flame hydrolysis or burning oxidation of silane compounds such as SiCl.sub.4, CH.sub.3 SiCl.sub.3, CH.sub.3 Si(OCH.sub.3).sub.3, Si(OCH.sub.3).sub.4, Si(OC.sub.2 H.sub.5).sub.4, and the like is carried out;
(2) A process in which an aqueous sodium silicate solution is subjected to ion-exchange to synthesize ultrafine colloidal silica, followed by Ostwalt growing; and
(3) A process in which an alkoxysilane such as Si(OCH.sub.3).sub.4 and Si(OC.sub.2 H.sub.5).sub.4 is subjected to wet hydrolysis at room temperature in the presence of an acidic or alkaline catalyst in a mixed solvent of water/alcohol [Journal of Colloid and Interface Science, 26, 62-69 (1968); Japan Chemical Society [9], 1503-1505 (1981); Kagoshima University, Technology Department Research Report [24], 115-122 (1982); and the same Report [26], 53-59 (1984)].
However, the silica particles obtained by Process (1) have a particle size of 200 to 500 nm, which is more than 10 times greater than the particle size of the particles obtained by Process (2). Also, this process, which is carried out by gaseous phase synthesis, can only produce silica with a low collection efficiency of about 50% at most, resulting in lower productivity. In addition, since association among particles may occur in the flame, it is difficult to obtain the so-called monodisperse spherical silica in which the particles having uniform particle size are each present in a mutually free state. According to Process (2), very fine and monodisperse silica having a particle size of 10 to 20 nm can be obtained in the state that particles are dispersed in water. However, in this process, which requires sodium silicate as a starting material, an H-type ion-exchange resin must be used or silicic acid serving as a nucleus having a particle size of 1 nm or less must be grown to a particle of the size of 10 nm; therefore the process is complicated and has poor productivity. Moreover, although the silica finally obtained can take the form such that particles are dispersed in water, it inevitably contains impurities such as Na originating from the starting sodium silicate and acid radical Cl or SO.sub.4 added at a step during the process for the purpose of adjusting the pH, as well as Al, in an amount of 10 to 1,000 ppm. Such a product can not be satisfactory for use in electronic materials. For example, as a use of the colloidal silica obtained by this process, there is known a polish used for silicon wafers for semiconductors or semiconductor wafers made of compounds such as GaAs or GdGa garnet. However, it has become clear that, as semiconductors have been made more highly integrated, the metals such as Na or Cl components in the colloidal silica may contaminate the wafers to adversely affect the performances of a device.
In contrast to these Processes (1) and (2), in Process (3), which is a process by which highly monodisperse fine spherical silica can be obtained as colloidal silica in the manner similar to Process (2), a high quality substance containing only a very trace amount of metals can be used as a starting material and solvent, and a volatile substance such as HCl and NH.sub.3 can be used as a catalyst. Accordingly, the impurities originating from the starting materials or the like can be in a very small amount, and the operation and apparatus for the wet hydrolysis can be simple and achieve high productivity. Thus, this is a good process. This process has also a feature that the resulting silica can be of porous structure.
However, when the colloidal silica is used as a polish for semiconductor wafers, it is required, in addition to the smallness in the impurity content, to have a particle size of 100 nm or less, preferably 50 nm or less. However, the colloidal silica obtained by the above Process (3) has a particle size of as large as 200 nm or more, and therefore there is a problem that it can not attain a desired polished surface.